For every parallel cast, store the payload as a property through a content modifier. Ex - p_content1, type expression, value ${in.body}

Use Join and Gather if you have used parallel multicast.

In gather use text/plan and concatenate as aggregation strategy in case of CSV files otherwise, it can be xml and combine.

After Gather step, make a process call the email.

In the local integration process for email use another content modifier to create headers and capture the payloads from previous content modifiers created in step 1. Add headers if there are multiple payloads in the same content modifier. Ex - name - h_content1,type - property, value - p_content1